The analysis of en-échelon veins

Abstract

SummaryMeasurements of en-échelon veins cutting small chevron folds in Carboniferous greywackes, near Hartland in Devon, allow the following new conclusions about naturally occurring second-order fractures to be stated. 1. Veins may be initiated between 10° and 46° to the shear zone in which they occur. Previous studies of en-échelon fractures have generally classified them as either tension gashes formed at 45° to an array, or Riedel shears at 15° to an array. 2. Veins at about 20° to 40° to an array probably occupy surfaces which are transitional between shear and extension fractures. 3. Proof of a component of shear, in addition to dilation, on many of the veins at less than 40° to an array is provided by crystal fibres at acute angles to vein margins, and third-order en-échelon veins in arrays which are parallel to and part of a set of second-order veins.